See Quantum with the Naked Eye

By Greg Kestin

Posted 02.07.18

NOVA

How can you train yourself to be a quantum detector? You can detect quantum mechanics all over—if you know how to look for it. In this episode of What the Physics?! find out how simply squinting at a lamp reveals the quantum nature of light.

Transcript

Seeing Quantum with the Naked Eye

Published February 6, 2018

Greg Kestin: What you’re seeing is actually quantum mechanics. You may have heard that quantum mechanics is the physics describing small things that you can never see — and will never see — in your everyday life. But if you know what to look for, you can see quantum mechanics all around you.

There’s nothing special about this dot. Look at a light — any light around you — and squint. You’ll see streaks shooting out of the top and bottom of that light, like this. But what are those streaks and where do they come from? How do we get light up here when the lamp is all the way down here?

Einstein showed that light is particles: photons. And those photons travel in a straight line. So if the photons of light are coming from here, then maybe I can block the streak by putting my hand between the streak and my eye. It doesn’t matter how close you get your hand to your eye; you can never block out the streak. It seems like the streak is actually jutting up behind your hand. The only way to block the streak is to block the light itself. So the streak must be coming from the lamp. But how is that possible if your hand is blocking the path from the lamp to where the streak appears to be, right behind your hand? Which begs the question: is it possible that the light from the lamp actually isn’t traveling in a straight line?

The answer is yes. And here’s an experiment that you can do to prove it. Get a laser pen and a piece of cardboard. Then slice a very tiny horizontal slit in the cardboard. Arrange it so that the laser light is going through the slit onto a wall. The laser light will actually go straight until it hits the little slit, and then it spreads out into this vertical pattern on the wall. Some of the light will travel straight until it reaches the slit and somehow bends and lands up here. If light can bend like this, then it makes sense that the streaks we saw earlier were coming from the lamp. But so far, none of this explains what the streaks — or what the laser dots on the wall — are.

If instead of laser light going through the slit you had water waves going through the slit, then there would be places on the wall where you would have waves crashing and other places where there are no waves. Those would line up exactly with the bright and dark places on the wall from our laser experiment. This pattern is called a diffraction pattern, and it’s always created by some kind of wave. This pattern of light on the wall implies that light is some kind of wave, a wave that can bend around corners, like water waves.

Here’s where it gets even crazier. What if our laser only shot one photon at a time? The first photon would land, possibly, here. The second photon would land, maybe, down here. But if we let a lot of photons build up, we’d see this wave pattern. In this way, photons are actually both a particle and a wave. That is quantum mechanics. When you squint at the lamp, the lamp acts like the laser, your eyelids act like the slit, and your retina, the group of light-sensitive cells at the back of your eye, acts like the wall.

Now, if you take your laser and shine it on a wall and then squint while looking at that laser dot, you’ll actually see a diffraction pattern. But that diffraction pattern isn’t on the wall, even though it looks like it is. The diffraction pattern is actually on your retina. You can also see this pattern by carefully squinting at some small white lights that you can find around the house. When you squint at a lamp, though, what you’re seeing is a bunch of these patterns all overlapped so it looks like a streak. These streaks are called diffraction spikes, and now that you know how to find them, I promise: you will see them everywhere you go. And just think: every time you see a streak, that’s actually particles of light, photons, acting like a wave, and you’re seeing quantum mechanics in action.